Hi Foz !

I think the easiest way is to load a custom battle on Scottish Highlands, choose either HRE or Scots as your own faction, deploy as close to your ally (the other faction) as possible and rush on over to them, then march overlapping them and mimick their moves. Give the enemy a similarly slow unit so hopefully you get in your position first. It is tricky because you may not get there first.

About the unit change, might be an idea ot learn how to do this anyway but for the screenshot I'd say you'll probably make quicker progress with a custom battle HRE + Scots as allies. I have'nt done that myself but can point you to this thread where I think I saw a mention of how to change factions for a unit. There was also a thread in the modding forum if I'm not mistaken. Whacker was also modding in some units for the Byzantines, I don't think he finished but was looking into it about a month back.