well what price range are u looking at? for best circulation its best to get a larger can an just use a spray bar to reduce the current while maintaining the same turnover rate. i got a 306 on a 20L shrimp tank an i like it i have it turned down as i havent put a spray bar on it but i could have it full blast if i did without having a mess.
for brand thats all up to u, my pref in order is 1 fluval great product great cs+1 #2 sunsun cheap works great 3# ehime great product bad cs(ill refuse to buy a good product with bad cs just how i feel about it), havent owned a rena never needed to as all my filters still work aquaclear=fluval and im sure u know about there hob's ;p.
so after YOU choose the brand and gph i try to say around 10x turn over rate so i never have debris build up but thats up to u and how you want your filter set up. if you go high gph and dont diffuse the water correctly you will have a whirlpool that will not be good, but its easy to slow a filter down vs increasing gph.
as for seeding the filter you dont need both on all you would have to do is put all the media into the new filter to seed that then after a month remove it an add what you couldnt fit with that in there, its pretty simple! good luck.
ps heres a lil trick for u if u want to make the intake more shrimp safe and not use a sponge, go to hobbylobby pick up plastic screen mesh cut to height of intake and the same outside lenght of the intake, pull off the intake roll it up slid it in works well