hmm, i think that tiger barbs AND angels would be asking for trouble. tiger barbs are pretty nippy and the angels long flowing fins would be very tempting.
if you were to get a pleco, i'd go with one of the smaller varieties such as the bristlenose pleco, the common pleco gets gigantic (24 inches) and IMO really needs a much bigger tank than a 75.
a huge school of tiger barbs and two bristlenose plecos OR a
red tail shark would be pretty neat, or a large school of silver dollars with the two BN plecos on the bottom would be good too.