introducing a new pleco! please read and answer!?

[Mark S]

hi, i have a 75 gal tank, bunch of fish, 1 huge piece of drift wood, 1 normal size piece of drift wood, 4 cave like accessories.
right now i have 2 female albino bristlenose pleco that ive had since 6/3/11 so their almost full grown.
i REALLY, wanna get more plecos. firstly i want to get 1 male albino bristle nose pleco, and then 1 pleco of anoter kind.
so how would i introduce them? or does it not matter? there are 4 local fish stores that i go to and 3 of them only have not mature ones so i wouldnt no if they were a boy or girl untill they get older unless one of u coiuld tell me how to tell their sex at a younger age if there is a way, and then the other store i will be calling tomorrow and asking if they have any albino bristle nose plecos and if they have any juvenille of adult males. and some other plecos that hopefully u could recamend to me to get that stays about 4-6 inches total and isnt agressive.

so basicly,,,
how is the right way to introduce 1 or 2 new plecos to a already established tank?
is there a way to tell younger plecos apart from boy and girl?
since i want 2 female albino bristle nose plecos and 1 male, what of 1 other pleco should i get?
OR could i have 2 female albino bristlenose plecos, 1 male albino bristlenose pleco, and then 1male and 1 female of another kind of pleco? an if so what otehr kind of peaceful pleco?
i put algea waffers in the tank everynight so they will have plenty of food.
what fruits or veggies should i be giving my plecos for a snack? and how should i prepare them to make it safe to put in the tank?! thank you to whoever answers my questions!