If you're going to alter guerilla then the most you should do is give them "all terrain as roads".
Their primary useage was for suprise attacks behind enemy lines.
They weren't all that stealthy compared to other units, but the small degree of extra stealthiness they did have would be more than compensated by being able to move through enemy territory relatively unhindered.
Only engineers were used for building roads, fortifications, etc., and that is what workers are for.
If you think a unit should exist for slipping invisibly in and out of enemy territory, and be able to build roads, forts, etc., while never giving away its nationality, then you should add the extremely expensive "Super spy" as a custom unit.
Also remember that what ever you do to a unit, your opponents will have access to them, as well.