The original question, should a painter stick with one product line through out a paint job. The simple answer is, in an ideal world , yes. If the question was can I use one manufacturer's product over another, or is it safe to use one manufacturer's product over another it gets a little more complicated...and...Brian, your right. The key word is cured.
If your using a PPG primer that is catalyzed and you want to use Dupont base, with SPI clear and if your asking if it will work safely, I would say Yes...under conditions. If the PPG primer is cured and I mean cured and prepped properly you can safely put Dupont base over top...the Dupont base, when totally flashed will accept SPI clear. This is no different than painting over top OEM finishes or over some other shops previous repair....If it wasn't safe, it would be mandatory for shops to out line what products they put on your vehicle when it was repaired or for the manufacturer to tell you which product line your vehicle was originally painted with...Example...For several years now the OEM has been using water born base coats and shops are repairing them with solvent base coats...they still get successfully repaired.
Then we get to sealers....I have posted several times what my feelings are about sealers...but...here goes again. Sealers, in my opinion, are not a necessary step. If they where, a color blend would not be recommended by paint manufacturers. If you know what your substrate is and that your substrate is consistent throughout your panel that you are painting and your substrate is prepped properly, sealer is not required and is just another coat that you can get dirt in. Sealer is an excellent product if your prep work is questionable, for example, if the manufacturer recommends that your substrate be prepped in 400 grit paper and the substrate is prepped in 320..a sealer will fill the 320 grit profile and eliminate sand scratches in your top coat. If your substrate is questionable...you don't know if part of it's lacquer, part acrylic enamel or base coat, sealer may offer you a "sandwhich" degree of insurance knowing that all of your paint is going over top of the same substrate...but...you don't know how the sealer is going to react with all of the different substrates underneath it. The difference is substrates underneath the sealer will contract and expand differently, absorb solvents and catalyst differently and could still cause "halos" or even wrinkling (if the sealer is shot over a non catalyzed product like lacquer or enamel or if the existing substrate has been oxidized to the point that it hasn't any properties of a freshly catalyzed product). The key answer here is know your substrate, if your unsure, test it by pepping the substrate, wiping it clean with a thinner, if your rag with thinner ends up being the same color as the substrate your wiping...chances are you have a weak substrate and it should be removed....if your still not sure, spray some sealer, primer, base or whatever over the prepped substrate, if it reacts by wrinkling, extra long flash or cure times you have a weak substrate and it should be removed. If your still unsure, because of it's age, fading or you just don't get a good feeling, remove it, if you don't, chances are your going to in the long run and having double the material bill. The days are long over of dusting lacquer primer over top of poor substrates to build a weak bridge before painting...if there not over...they should be...that's just a poor repair or method of trying to get to a respectable finish.
Hope this helps