Large roll-up company, has small subsidiaries all over the country. Has 5 or 6 prevailing wage plans for various employee groups, various part of the country.

Are there some general (ie, obvious) reasons why these plans could not be combined into one plan?

You can "roll" the plans into one plan, but you would have a prevailing wage formula for each subs respective Davis Bacon employees. As long as the formulas pass non-discrimination (401(a)(4)), I don't see a problem with putting them into the same document. You could also submit the document, after the plans are combined, for an IRS determ letter which could at least provide some assurance on the qualification on the document language and if opted the plans operation (Schedule Q to form 5307).