BB, i tried your suggestion out and here's my thoughts:
aiming for spies as the Scotts or Picts slows down considerably the rushing flow if you aim for them too early; this is because you divert resources and time from making troops and boats to tech up and make spies early-on that resources are limited and so your rate of early expansion is slower than it otherwise could be.

Of course once you start the ruh you can move slightly quicker as you say, yet, the early time lost finds the Mercians and the Saxons more advanced and powerful than it would otherwise.

Perhaps it may be worth it to go for it as the Welsh that start with a fort (or was it keep?) in Wales and hence it takes less time and resources for them to get them.

Rushing as the Scotts/Picts/Welsh i make use of Clansmen/Celts as much as possible. I garrison newly acquired provinces and put taxes at lowest for the first few turns and build the early happy buildings aiming to release the garrisons asap to go to teh front line and keep attacking. While the Celt warriors cost a little bit more than the clansmen and they are slightly less suitable, they are als more in number and hence they somewhat make up for it. Clansmen are just too good and too cheap hence perfect for rush.

Of course there are other strategies and approaches than a straight all out head on attack