I have had all my indoor cats declawed. And at different ages. I have never had a problem and they have healed quickly(the last 2 done by laser). I might mention, 1 was about a year old and weighed 15 - 20 lbs when declawed and 1 was over 2 y.o. and weighed about 20 lbs when declawed. 2 were done at 6 mos. with 1 ending up weighing just under 30 lbs. and the other is a normal sized cat.
We had 2 cats (1 was terrified of big dogs) when we got Roxy. They were curious but by no means thrilled. They hissed and swatted and growled but noone got hurt. They have adjusted well and Roxy even rounds them up when it's time to come in for the nite. I know alot of it is show, because when they don't know I am watching, I have caught them giving each other baths. I have even seen them sleeping together. If you have unneutered males, they are probably protecting their territory. Give them time and you can always use a squirt gun or bottle of water on the cats - they hate that. Good luck!