If you want to be anal, or are noticing some slowdown, a good habit is to use the lotdebugger to clear gossip periodically. At the least, clear it from the dead. I clear about every generation.
I learnt something useful today, thanks. I delete the sims manually, and so the gossips and all the memories go with them, but it seems much easier and faster with the lot debugger, I did not know about this feature. There is no more need to remove the whole sim if I understand. The relations and memories are automatically nuked for the dead ones, right ?
I don't use the clean templates.Like you (dirtandpretty ) I created my own custom hoods pre-populated with my own custom NPCs and made a backup of it for all my new hood needs.
With all subhood including university the total is 653 characters at the start. With the fixes available on this site the increase is under control and I don't encounter any problem with neighborhoods of more than 1200, it covers about 5-6 generations of playables with about 30 families. After that you could always delete the deads after 2 generations, but I don't do it myself because generally the hard drive or the computer crashes before I have to do (never understood how some people can keep their PC alive for more than 2 years, not speaking of hard disks but this is another story).
So you don't need to worry