For our big tank we are going to use colorquartz for sand, as you can get colors, (we want black sand) for a lot cheaper than moon sand (or something similar). You can have a place the builds/repairs pools special order it for you, in my area, it runs about $20/50lb bag and 2-3 bags should be plenty for you. The T grade is supposed to not be as fine as sand (ie it won't get into your intakes, but still has a sand look) but they also have an S grade that is very fine. Personally with any kind of cichlid I would use some sort of sand because it is easier to maintain, and especially with big cichlids, or any big fish, you're going to want it to be as easy to keep clean as possibly because they are so messy. Here is some more info on the colorquartz:
http://solutions.3m.com/wps/portal/3M/en_US/ColorQuartz/HomePage/Products/Crystals/
As far as filtration, the bigger the sump the better IMO, because then you'll have room for more media. A lot of times you can find a used 55 on craigslist for cheap, I have gotten a used 55 from freecycle.org for free though, so you might want to look into that, I think a 55 gallon would be sufficient.
As far as stocking goes, I don't know if you'd be able to house EIGHT big fish in a 125, that seems overstocked even by my standards and I overstock everything. I would say 2 oscars if those are the fish you like to keep would be max, because you are supposed to keep even numbers, and 4 would be too many IMO. I don't know what other kind of cichlids you like, but in a 125 I could maybe see 2 oscars, and 2 medium cichlids, such as jack dempseys. But unless you pick a type of fish I don't see hoe you could get 8 of them in there. Not to mention, with cichlids, they all need their own territory of they will start to bicker and someone will end up dead, because that isn't a whole lot of space when you split it up 8 ways.
I would also suggest one of these:
http://www.cichlid-forum.com/articles/diy_python.php
as for tankmate suggestions/possiblities to go with you oscars, as stated abovem you could maybe do a jack dempsey or two, or some convicts, some loaches (make sure they are bigger than the oscars because they grow slow, they need to be in a group of at least 3, 5 would be better, and have hiding places for them), any type of bichir would be an interesting tankmate (also make sure it is bigger/same size as your oscars). Also a school of silver dollars would be an interesting addition. You could add a synodontis catfish to clean up uneaten food (you will still have to feed it, it just help keep the substrate a bit cleaner since the oscars are messy eaters). You could also have a smaller pleco in there, but personally I would not get a common pleco or anything that gets to be that large...
If it were my tank and I already had the 2 oscars, and wanted to pack it as full as I could get it, I would get a bichir (p. senegal is the most commonly found):
http://www.thetropicaltank.co.uk/Fishindx/poly-sen.htm
3-4 Silver dollars:
http://www.fishlore.com/Profiles-SilverDollar.htm
a synodontis catfish:
http://planetcatfish.com/catelog/species.php?species_id=89
that isn't 8 big fish, but it's 8 fish, and it's still overstocked, which means you would have to do water changes more often and have a larger bio-capacity in whatever filtration you choose, I would still think a 55 gallon tank/container would be enough for a sump if you had LOTS of bio in it. Just remember that maximum stocking = maximum tank and filter maintenance....
