What is the intended use of your projects (indoor, outdoor)?  Are the forms 3-dimensional?

How sturdy is the papier mache?  Will the mosaic be tiles or stones (heavy materials)?  Do you intend to grout the joints?

If the forms are sturdy and completely dry, I would waterproof them with several coats of shellac or polyurethane. Be careful to coat the entire form, including the bottom.  Then, if the form seems substantial enough to you, mosaic directly onto the form.  (Yes, I don't see why you couldn't use plaster strips as long as the form was totally sealed, but run the strips through your fingers to eliminate excess water.)

Two warnings:  Do some research on the adhesive you're using for the mosaic work and make sure it doesn't conflict with your sealer.  Also, if you're using tiles or stone, weight and balance will become issues.  Pay attention to where the weight is.  You don't want too much weight at the top if the base isn't large enough or stable enough to support it.

I would be really nice if you posted a photo of your project here in the gallery, and let us know it's there.

Good luck with your project!

Sue

I don't know that using the balloon base would adversely affect your work as long as you made it substantial enough to carry the weight.  And if you sealed it prior to adding the mosaic, that should add to the firmness.

This sounds like a good way to determine what your limitations might be, or how you could overcome them.

Sue