My DD has just started Y1 and we've spent the last year taking her to lots of 5th birthday parties, as there were a lot of people who invited the whole class, and she got invited to plenty of smaller parties too.
We have a class WhatsApp group and that's how people did invites when they were inviting the whole class. Advance notice is good (maybe 4-6 weeks?) but the further in advance, the more you'll need to chase for RSVPs nearer the time.
In my experience, people will ask politely if they can bring a sibling when they RSVP. If it's a hall party I always agree to siblings but you do have to prepare extra party bags or be very careful when giving them out (we had an issue with this at DD's recent birthday party!)
Absolutely no-one has done "drop and run" at this age, in all the 5th birthday parties we've hosted and attended (DD is our second child), and I think it's very unlikely, especially if your son is one of the first among his friends to turn 5, as most kids will only be 4 and won't know you very well. It is an opportunity for the class parents to get to know each other a little bit. The small talk aspect isn't for everyone but it is helpful to be friendly with a few of the class parents, I find.
Bouncy castles are fun but the kids can get a bit crazy and hurt themselves or each other, so you will need someone keeping an eye - some parents are more "on it" than others. It's best to ensure you have other things to entertain the kids. The most successful hall parties we've attended have had a children's entertainer, doing games, music, magic tricks or bubble show, that kind of thing.
Re food - I wouldn't be ordering pizza for that number of young kids, it would cost a fortune and a lot would be wasted. (We did domino's for DS's 9th birthday party but it was smaller numbers, they did sport beforehand, and they were hungry so it all got eaten!) Simple cold buffet party food will be fine.
Hope that helps!