Can you not reduce the concentration of your ammonium molybdate? You control film thickness by precursor concentration and spinning speed. Almost any surfactant at 1% concentration or less that’s compatible with your molybdate will probably do the job. I‘d trial washing up liquid to start or a non-ionic surfactant such as Nonidet P42 (sometimes called Igepal) if you want to spend some money. Substrate cleanliness is the crucial parameter here - I assume you can filter all solutions to 0.2 microns or better and can work under Class 100 conditions.