A LRM20 has its place in an (balanced) Atlas. Here you have a lot of tonnage to spare with few missile tubes. If you still want some SRM backup, you need an LRM20. You only got an 10 tube launcher in there, which splits up an LRM15 in 10-5. If your target has AMS, most missiles get shot down.
So you should only go for an LRM10 or LRM20
Of course a LRM Atlas is not the best to begin with