Yes you can upload to gallery as much as you want. Afaik the gallery is indepenent from the forum and the avatars.
So:
-rather not post attachments on the forum: use upload services for that
-keep avatars as small as possible (compression i mean, not actual size) or select an external avatar (upload service)
-you can post as much as you want in the gallery, as far as i know at least.
-i haven't seen any limits about posts (post text size, ammount of posts, etc), so i set the limit for that pretty high
-for signatures same as for posts, no limits that i know so I set the max text and height to a fair ammount
All clear now?