Question [Q] About kernels and basebands

I've searched the forums but to no avail, so I'll just ask this here. I'm interested in changing the ROM of my Xperia X8 (E15a). With S1Tool I checked whether I could unlock the bootloader of my device, however the certificate is not recognized so that's a no-go for me. For this reason I'm stuck with stock kernels only.

Here is my device info:
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Model number: E15a
Firmware version: 2.1-update1
Baseband version: M76XX-TSNCJOLYM-53404006
Kernel version: 2.6.29 SEMCUser@SEMCHost #1
Compilation number: 2.0.1.A.0.47
Here's when it gets confusing. All the ROMS I find here on xda say that I should use a baseband that ends in 015 and if I don't have that baseband, I should update with PC Companion to the latest one. PC Companion says that my device is already up to date, so I can't get another baseband. So I thought of trying flashing with FlashTools a baseband that actually ends with 015. However I read that that basebands work with specific kernels, so my best guess is that I should find a baseband for kernel 2.6.29, right? Well, I'm out of luck cause I can't seem to find that thing.

So, basically the question is: How do kernels and basebands are related and what should I do if for instance I want to install this ROM: http://forum.xda-developers.com/show....php?t=1079463... should I flash a older kernel for which a baseband with 015 does exists or what?

Thanks in advance