David,
As you say, it appears that Ancestry users will not be able to use GEDmatch triangulation - even with some accomodation for a different format of data. I didn't understand what data was available when I jumped in here, so I stand corrected. I suppose the good news is that there are other useful utilities based on raw DNA to fill the gap. I've already mentioned "matches in common". There's also a chromosome browser in the one-to-many comparison utility that shows what segments you have in common.

Not sure how many from each company. Most are from 23andMe. There are something over 20,000 total DNA kits in the GEDmatch database. Some are duplicates where people have tested from more than one company. Some are 'phased' kits that were generated on-site from a child and one or both parents.

John