Sleeves are not bad. They are used quite often in structural repair and IIRC, there's a diagram in the GCR about sleeving.

If you're concerned about the length of the weld, then notch the sleeve on both ends with 60 deg. notches. That plus rossette/plug welds will make a very strong joint.

As far as the same size tubing requirement, all required tubes must meet the minimum requirement. Sleeves typically do not make the tube thinner