Sevs will eat anything they can fit in their mouths but I dont think the tigers or congos would be a problem as they will get bigger than the sevs mouth. As said the clown loaches need groups but I think that there are other loaches better suited to a 75 than clowns with a max size of 12 inches each one would take up a lot of room in your eventual stocking.
What size tank is this is it a 4ft tall or is it a 6footer? I think if its a 4 just go with one sev. Any ideas which one you want to get? I really like heros notatius (sp) at the moment thinking of one for my future big tank.
Also with the BN plecs, is this preference or because you want small plecs? If you want these because you like them go for it but if you just want some small plecs there are tons of them that are just as easy to find that get to roughly the same size - if you look in the plec section here there is a list of plecs under 8 inches all of these are suitable for your tank. Also you could look at planetcatfish as they have a full library of all the L numbers including sizes.
Also I think you could have more cichlids in that tank if you wanted as well, how about some small eartheaters - like a small group of tajos or cupid cichlids these would work well with your sev I think and also your other fish. So maybe your stocking could be
1 Sev
4 Red head taujos
5 Yoyo loaches (or similar - kubutia, histronica,)
2 BN plecs (or similar sized if you like some others)
10 Columbian tetras (just my preference I prefer one big school)
hope its helped Wills
ps what kind of filtration do you have?