This isn't precisely an answer to your question (welcome to HBT, right?) but you could still do low OG five gallon batches with a single cooler or "normal" beers using both coolers. You could also make some no-doubt-stellar partial mash batches with a single 3 gallon cooler. You would simply use as much grain as possible in a 3g cooler and then make up the rest of your gravity with dried or liquid malt extract. There are lots of great software options out there to help you with this sort of thing (I use beersmith). Great beer and no need to buy extra, smaller fermentors.
As far as a false bottom, there are probably machined options available but it would probably be easiest and cheapest to just got with a wire mesh braid or copper/pvc with slits in it. There are tons of posts on HBT illustrating how to do that for any mash tun size. Once you get used to all-grain you can spend the money you save on upping to a ten gallon tun with a machined false bottom
You will probably need a smaller yeast starter- check out mrmalty.com for calculations. For most applications you could probably just use a single dried packet or smack pack for almost any non-crazy-high-og beer and get good results.
I'm sure others will chime in with more technical responses, but I'd say keep it super simple and cheap, see if you like the increase in time/effort required for all grain and then you'll know just what to do when you upgrade!
edit: i get my SS fittings from bargainfittings.com, as do a lot of folks here. You'd be just fine with brass for application if you want to save some money, but you could always transfer the SS ball valve/etc to a bigger mash tun in the future