So I have a Xk x6 transmitter and I want to get an eachine assassin or something like that so my question is can I somehow bind my Xk x6 transmitter with other quads that are not Xk products
Although those TXs look and work well they are quite limited.
You'll often see RTF packages with the FS-i6. That TX has a 20 model memory (all settings can be individualized/specialized). You just need to buy one of the 2 available RXs for each model you set up.
Do note the Eachine i6 FW is a bit restricted. There is no way to set up/use the FS-iA6B's RX PPM mode. Many will flash it with the real FlySky FW.