Can it be used to serve the mission of an exchange?
Yes, for example you can send some LN-BTC if (and only if) someone else will send you some LN-LTC. If swapping LN-BTC with LN-BTC is possible, then swapping LN-BTC with LN-ALT is also possible, as long as both parties agree on the price.
Like - Is it possible to create hundreds of lightning networks over a single platform?
Why not? One HD wallet per user is enough to handle all ECDSA keys for all coins. A lot of coins are very similar to BTC, Lightning Network could be used almost everywhere, as long as 2-of-2 multisig, timelock and similar stuff works.