Latency wise it seems okay for llm training to put them higher than Starlink to make them last longer and avoid decelerating because of the atmosphere. And for inference, well, if the infra can be amortized over decades than it might make the inference price cheap enough to endure additional latencies.
Concerning communication, SpaceX I think already has inter-starlinks laser comms, at least a prototype.
There are plenty of data centers in urban centers; most major internet exchanges have their core in a skyscraper in a significant downtown, and there will almost always be several floors of colospace surrounding that, and typically in neighboring buildings as well. But when that is too expensive, it's almost always the case that there are satellite DCs in the surrounding suburbs. Running fiber out to the warehouse district isn't too expensive, especially compared to putting things in orbit; and terrestrial power delivery has got to be a lot less expensive and more reliable too.
According to a quick search, StarLink has one 100g space laser on equipped satellites; that's peanuts for terrestrial equipment.