There is a LOT of misinformation going around about this topic out on the internet right now. I present to you the scientific reasons why you should NOT mix acrylic brands and systems.
Monomers and polymers are formulated to work together as a pair. To get the optimum results you should use the full system of any brand. Each systems' powder has a certain amount of catalyst in it, that amount is precisely required to complete the polymerization (cure) of the enhancement. Each system has different amounts of catalysts and different reaction speeds. Some brands put the catalyst in their powder and the initiator in their liquid. Others do the opposite. Unless you work for the chemistry division for that company you are not going to know what is in a product and in what concentrations.
"All powders contain various ingredients and levels of ingredients. One very important ingredient with most powders is Benzoyl Peroxide (BPO). BPO is one ingredient that is responsible for controlling the curing of your monomer. Too little means a slower cure time (in some instances a nail that never fully cures). Too much and you will 'shock cure' the enhancement. This is why mix ratio is an important part of working with your L&P system. The wetter of a mix you use, the less BPO. The drier of a mix you use, the more BPO. This can radically affect the performance (and more importantly) the safety of the application." (Quoted from Sam Sweet, CND Educator). This is why mix ratio and using the same brand is so very important.
In addition it can be dangerous to mix monomer and polymer systems. It may SEEM fine on the outside, but you are putting your client at serious overexposure risks that may not manifest themselves for months after. You could also be found legally negligent - a client could very easily sue you if they develop a reaction and your insurance company will NOT cover you because you were not following manufacturer's instructions not with any of the manufacturers back you up. ***It is not always obvious to the naked eye when things are not quite right.*** Yellowing, premature break down, cracking, lifting, overexposure...the list goes on and on. If you like Brand A liquid, use their powder. If you like Brand B powder, use their liquid. Safety and security for clients should NEVER be negotiable.
One last thing I found on the SalonGeek message board when researching this topic - I have not fully researched this fact so I am presenting it to you more as a thinking point that I find interesting and very plausible - "Most of the companies that say their polymer is 'universal' are the same companies that do not have an R&D facility and to me seem more interested in making a quick sale then investigating the reality of the situation."