Welcome to MFK and back to fishkeeping Olaf!
Ill just run down your questions and answer best I can:
Yes, you should be set with filtration.
Obviously, the size of the fish will determine how many you can have in your tank.
Most EBJDs are frail and hard to grow out in a community of fish other than tetras, mollies and the like, in my experience. SOmetimes GTs suffer from the same stomach problems that EBJD do and are prone to some form of parasite or bloat, not sure which it is but I seem to always see "Why is my GTs poop white" threads. I dont have personal experience with them so this is just based on the threads I see on here.
Carpintis...they are usually mislabeled as "Green Texas" or "Blue Texas", but in reality they arent Texas cichlids. That title is (or should be) reserved for the true Texas cichlid, Herichthys cyanoguttatus, the only native cichlid to the USA.
Herichthys carpintis come from mostly Northern Mexico and are available in several variants: Lahillas, Escondido, Chairel, Vontehillo & a few other rarer variants. The cyaonguttatus is more aggressive than the carpintis and is typically more blue with way smaller splotches. Both could live in a 75 for life and you could probably keep a pair in one as well, although cyanoguttatus can get about 12" max while carpintis get around 8-10".
Convicts make great smaller target fish or smaller tank companions to larger cichlids, as well as Honduran Red points and sajica, mostly because these 3 fish are aggro, tough and can hold their own against most fish 4x their size.
Salvinis are cool but very aggro and males can get up to 8".
Dwarf pikes are cool and stay rather small for a pike. Heres a link about them:
http://www.cichlid-forum.com/articles/dwarf_pike_cichlids.php
If you go with your "perfect world" choices, Id grow out the EBJD & GT to at least 3 or 4" before putting them together (or maybe purchasing some that big if its an option) and definitely before putting them with a convict, especially a male. Im not 100% sure that a GT or Con wouldnt beat up an EBJD regardless of their size, but Im sure others here with experience with EBJDs & GTs will offer their views. Mine is just 2nd hand from reading about the combo on the web and more so on here.
One other choice, if you have a local LFS that will take trades, is to get a bunch of fry of whatever you think youd like or that would live in a 75 full grown, grow them out and just remove problem fish as they arise. Ive had a lot of success in doing so and helped my old LFS in doing so, providing him with bigger fish to sell than he had originally.
Good Luck with whatever you choose!