Adding Train Foundry to an existing Factorio 2.1 Space Exploration save should be fine, though I haven’t tested this combination yet. My main uncertainty is placing a foundry in orbit: it’s a large building that creates its own rails, and I haven’t checked how that behaves on Space Exploration’s platforms. I’d recommend backing up your save before trying that.
I’m not aware of a direct conflict with LTN. Train Foundry builds trains; LTN manages deliveries. With the blueprint version, you could try building a train whose schedule sends it to an LTN depot, but I haven’t tested that workflow with LTN yet
