I bought Minecraft from the Mojang Store. A friend of mine got Minecraft from the Windows Store. Can we play together on online servers or is that not possible?
No, I don't think you can use Hypixel with the Bedrock Edition
Fans are probably working on plugins that are supposed to enable playing on Java servers with the Bedrock version, but whether this will really be possible at some point is in the stars.
In any case, this is currently not possible.
Take a look at Geyser, it works there. You only need a Mojang account if you don't host the server yourself or you play on a cracked server e.g. Blocksmc.com. (I tried it myself, it worked very well)
And since you still need a Java account, the plug-in for public servers is completely unnecessary.