You can import any pack into YEM at any time. Means, you can have installed packs for Genos and SX900 at the same time. If you have packs that are made for specific keyboard, then it's a good practice to have them properly named (i.e. "Latin-SX900"), so you can distinguish them.
While you're in
My Packs view, you can remove pack from YEM by selecting the pack and then click
[>] button at the top of the YEM (Delete option will appear).
If both (Genos and SX900) packs contain the same voices and styles, then it doesn't matter which one you use to create "install" pack for Genos (or for SX900) -it will sound the same. But keep in mind: style for Genos might use voices which are only built in Genos. In this case such style won't sound properly on SX900 -to get good result, you will need to "revoice" such style.
My question is why are the SX900 packs showing up in the Genos YEM page?
There's no Genos (or SX900) YEM page... by selecting particular keyboard in
Install Target column, you only define for which keyboard you wish to create install pack. YEM sees all packs equally, because technically, there's no difference between (for example) Genos or SX600 pack. By selecting content (voices & styles) for install pack,
you need to pay attention what pack you use as source (so you make sure styles will sound properly).
To remove particular keyboard in
Install Target, you select desired keyboard and click on
[>] button at the top of the column.
This are basic YEM functions which you should learn and I highly recommend you take time and read YEM manual.
Bogdan