Very simple

Remove the plastic clip/joiner from the Kato Unitrack.  It's a snap fit.  Build up a roadbed under the Peco track so that the tops of the rails match.  Connect the Peco to the Unitrack with a standard rail joiner.

Joining Unitrack to Peco

Thank you highway70 -  that is simple, It's my first foray into the kato system and our local shop doesn't stock it so I wasn't aware of the fact that the plastic clip/joiner could be removed.Thanks.Tom 

Rail cross section

One thing I have not seen mentioned in other threads is that the cross sections of the rail used by the various companies differs.  Even when the nominal rail height the same (for instance code 80. The sizes of rail heads, flanges and webs can be very different.  There may even be very slight differences in the actual rail height.  None of these differences preclude using the different brands together. 

When connecting different brands, carefully observe the connection and make adjustments to the best alignment.  This is usually all that is needed, although sometimes it may be necessary to lightly file some of the rail heads  to smooth the transition.

Because of the different flange sizes rail joiners can be a problem.  For instance Atlas code 80 has a wider flange than Peco code 80.  Because of this Atlas joiners are a little loose on Peco track while Peco joiners are a little tight and very difficult to place on Atlas track.  

Also note that you can use Peco code 55. It is a pseudo code 55 that is compatible with code 80 track, but not real code 55 track. 

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Another thought:

I don't like soldering rail joints for electrical conduction.  My experience is that it is not reliable.  Soldering feeders to every rail section works better for me.  Also if the joints are not soldered, revisions to the layout or replacement of broken or  defective switches is easier. I sometimes solder joints to help maintain proper alignment. Although I have not done it, I have seen recommendations to solder the  joints in flex track to be laid an a long  curve prior to laying the track to help avoid a kink at the joint.

Unitrack/Peco

Thank you highway70 for your comments - all taken aboard. Looks like working well for me but as you say the rail joiner is a bit of a problem. The peco ones are too tight but I got some generic ones from the hobby shop and they are doing the job. Tom