Yes, you can parallel charge lead acid batteries, as long as they are the same age and condition. If one battery is weak or old it can draw all the charging power. In general its hard to get them all to charge evenly - you will be putting most of your charge into the weakest battery. If you use a really big charger on a bunch of small batteries you run the risk of cooking the lowest battery before the charge equalizes.
Lead Acid batteries don't have a high self discharge, its not necessary to keep them on a trickle charger unless you are leaving them for over a year.
A lot depends on the batteries you are using, check the manufacturers sheet and see what the maximum charging current they allow and size your charger for that.
P.S. I was using some Hawker Genisis batteries a few years ago and under "maximum" charging current they said "what ever you can afford" They gave an example of charging a 13AH battery at 200A!
Tom