diff --git a/code/modules/mob/living/carbon/xenomorph/Evolution.dm b/code/modules/mob/living/carbon/xenomorph/Evolution.dm
index d8767ffce87a..332f8d1778dd 100644
--- a/code/modules/mob/living/carbon/xenomorph/Evolution.dm
+++ b/code/modules/mob/living/carbon/xenomorph/Evolution.dm
@@ -134,11 +134,13 @@
 			qdel(new_xeno)
 		return
 
-	switch(new_xeno.tier) //They have evolved, add them to the slot count
-		if(2)
-			hive.tier_2_xenos |= new_xeno
-		if(3)
-			hive.tier_3_xenos |= new_xeno
+	var/area/xeno_area = get_area(new_xeno)
+	if(!is_admin_level(new_xeno.z) || (xeno_area.flags_atom & AREA_ALLOW_XENO_JOIN))
+		switch(new_xeno.tier) //They have evolved, add them to the slot count IF they are in regular game space
+			if(2)
+				hive.tier_2_xenos |= new_xeno
+			if(3)
+				hive.tier_3_xenos |= new_xeno
 
 	log_game("EVOLVE: [key_name(src)] evolved into [new_xeno].")
 	if(mind)
@@ -355,24 +357,27 @@
 
 /mob/living/carbon/xenomorph/proc/can_evolve(castepick, potential_queens)
 	var/selected_caste = GLOB.xeno_datum_list[castepick]?.type
-	var/free_slots = LAZYACCESS(hive.free_slots, selected_caste)
-	if(free_slots)
+	var/free_slot = LAZYACCESS(hive.free_slots, selected_caste)
+	var/used_slot = LAZYACCESS(hive.used_slots, selected_caste)
+	if(free_slot > used_slot)
 		return TRUE
 
-	var/burrowed_factor = min(hive.stored_larva, sqrt(4*hive.stored_larva))
-	burrowed_factor = round(burrowed_factor)
-
 	var/used_tier_2_slots = length(hive.tier_2_xenos)
 	var/used_tier_3_slots = length(hive.tier_3_xenos)
-	for(var/caste_path in hive.used_free_slots)
-		if(!hive.used_free_slots[caste_path])
+	for(var/caste_path in hive.free_slots)
+		var/slots_free = hive.free_slots[caste_path]
+		var/slots_used = hive.used_slots[caste_path]
+		if(!slots_used)
 			continue
-		var/datum/caste_datum/C = caste_path
-		switch(initial(C.tier))
-			if(2) used_tier_2_slots--
-			if(3) used_tier_3_slots--
+		var/datum/caste_datum/current_caste = caste_path
+		switch(initial(current_caste.tier))
+			if(2)
+				used_tier_2_slots -= min(slots_used, slots_free)
+			if(3)
+				used_tier_3_slots -= min(slots_used, slots_free)
 
-	var/totalXenos = burrowed_factor
+	var/burrowed_factor = min(hive.stored_larva, sqrt(4*hive.stored_larva))
+	var/totalXenos = round(burrowed_factor)
 	for(var/mob/living/carbon/xenomorph/xeno as anything in hive.totalXenos)
 		if(xeno.counts_for_slots)
 			totalXenos++
diff --git a/code/modules/mob/living/carbon/xenomorph/Xenomorph.dm b/code/modules/mob/living/carbon/xenomorph/Xenomorph.dm
index 368dded9c17e..90c22e19c483 100644
--- a/code/modules/mob/living/carbon/xenomorph/Xenomorph.dm
+++ b/code/modules/mob/living/carbon/xenomorph/Xenomorph.dm
@@ -499,11 +499,7 @@
 	// Only handle free slots if the xeno is not in tdome
 	if(!is_admin_level(z))
 		var/selected_caste = GLOB.xeno_datum_list[caste_type]?.type
-		var/free_slots = LAZYACCESS(hive.free_slots, selected_caste)
-		if(free_slots)
-			hive.free_slots[selected_caste]--
-			var/new_val = LAZYACCESS(hive.used_free_slots, selected_caste) + 1
-			LAZYSET(hive.used_free_slots, selected_caste, new_val)
+		hive.used_slots[selected_caste]++
 
 	if(round_statistics && !statistic_exempt)
 		round_statistics.track_new_participant(faction, 1)
@@ -706,16 +702,7 @@
 		hive.remove_hive_leader(src, light_mode = TRUE)
 	SStracking.stop_tracking("hive_[hivenumber]", src)
 
-	// Only handle free slots if the xeno is not in tdome
-	if(!is_admin_level(z))
-		var/selected_caste = GLOB.xeno_datum_list[caste_type]?.type
-		var/used_slots = LAZYACCESS(hive.used_free_slots, selected_caste)
-		if(used_slots)
-			hive.used_free_slots[selected_caste]--
-			var/new_val = LAZYACCESS(hive.free_slots, selected_caste) + 1
-			LAZYSET(hive.free_slots, selected_caste, new_val)
-
-	hive.remove_xeno(src)
+	hive?.remove_xeno(src)
 	remove_from_all_mob_huds()
 
 	observed_xeno = null
diff --git a/code/modules/mob/living/carbon/xenomorph/xeno_defines.dm b/code/modules/mob/living/carbon/xenomorph/xeno_defines.dm
index 1de164fccc85..a74fa7413804 100644
--- a/code/modules/mob/living/carbon/xenomorph/xeno_defines.dm
+++ b/code/modules/mob/living/carbon/xenomorph/xeno_defines.dm
@@ -264,8 +264,8 @@
 		/datum/caste_datum/hivelord = 1,
 		/datum/caste_datum/carrier = 1
 	)
-	/// Assoc list of free slots currently used by specific castes
-	var/list/used_free_slots
+	/// Assoc list of slots currently used by specific castes (for calculating free_slot usage)
+	var/list/used_slots = list()
 	/// list of living tier2 xenos
 	var/list/tier_2_xenos = list()
 	/// list of living tier3 xenos
@@ -479,6 +479,12 @@
 	else if(xeno.tier == 3)
 		tier_3_xenos -= xeno
 
+	// Only handle free slots if the xeno is not in tdome
+	if(!is_admin_level(xeno.z))
+		var/selected_caste = GLOB.xeno_datum_list[xeno.caste_type]?.type
+		if(used_slots[selected_caste])
+			used_slots[selected_caste]--
+
 	if(!light_mode)
 		hive_ui.update_xeno_counts()
 		hive_ui.xeno_removed(xeno)
@@ -789,30 +795,31 @@
 		),
 	)
 
-	var/burrowed_factor = min(stored_larva, sqrt(4*stored_larva))
-	burrowed_factor = round(burrowed_factor)
-
 	var/used_tier_2_slots = length(tier_2_xenos)
 	var/used_tier_3_slots = length(tier_3_xenos)
-	for(var/caste_path in used_free_slots)
-		var/used_count = used_free_slots[caste_path]
-		if(!used_count)
-			continue
-		var/datum/caste_datum/C = caste_path
-		switch(initial(C.tier))
-			if(2) used_tier_2_slots -= used_count
-			if(3) used_tier_3_slots -= used_count
 
 	for(var/caste_path in free_slots)
-		var/slot_count = free_slots[caste_path]
-		if(!slot_count)
+		var/slots_free = free_slots[caste_path]
+		var/slots_used = used_slots[caste_path]
+		var/datum/caste_datum/current_caste = caste_path
+		if(slots_used)
+			// Don't count any free slots in use
+			switch(initial(current_caste.tier))
+				if(2)
+					used_tier_2_slots -= min(slots_used, slots_free)
+				if(3)
+					used_tier_3_slots -= min(slots_used, slots_free)
+		if(slots_free <= slots_used)
 			continue
-		var/datum/caste_datum/C = caste_path
-		switch(initial(C.tier))
-			if(2) slots[TIER_2][GUARANTEED_SLOTS][initial(C.caste_type)] = slot_count
-			if(3) slots[TIER_3][GUARANTEED_SLOTS][initial(C.caste_type)] = slot_count
+		// Display any free slots available
+		switch(initial(current_caste.tier))
+			if(2)
+				slots[TIER_2][GUARANTEED_SLOTS][initial(current_caste.caste_type)] = slots_free - slots_used
+			if(3)
+				slots[TIER_3][GUARANTEED_SLOTS][initial(current_caste.caste_type)] = slots_free - slots_used
 
-	var/effective_total = burrowed_factor
+	var/burrowed_factor = min(stored_larva, sqrt(4*stored_larva))
+	var/effective_total = round(burrowed_factor)
 	for(var/mob/living/carbon/xenomorph/xeno as anything in totalXenos)
 		if(xeno.counts_for_slots)
 			effective_total++