there might be a limit, but you have to download a lot to fill it. (I've had over 11 GB installed once, and even though the game was slower than a couple of snails, it still ran). it depends on how much your computer system can take. (I've got an 3 year old somewhat good laptop, all EP/SP's installed and a few gigs of dl's - and also most of the graphics on high. I get messages almost each time I play about minimum memory limit reached or something (512 MB), but I just unclick it and continue playing
Exactly which downloads don't show up? if it's maxis recolors, you probably need the newest CEP download over at MTS2. it enables recolors for maxis meshes without recolor possibiilities. (just search for CEP).
if it's clothes/hairs/custom furniture, you probably miss some meshes. just go back to the sites you found the hairstyles/clothes, and read what it says about meshes. some follows the download, and some you have to download elsewhere.
some downloads need a certain EP (expansion pack) or SP (stuff pack) to work, so make sure you've got the EP's you need for the download.
there's also the possibility that they've hid somewhere you didn't think they would be.
hope this helps