You have to choose one school, you can't be Hanafi and Maliki, like how you can't be Deobandi and Barelwi.
sorry you're comparing apples and oranges.
Barelwi and devbandi is an issue of Sunni 3aqidah vs a cocktail of non-Sunni 3aqaid
Maliki and Hanafi are issues of fiqh within Ahlus Sunnah. The reason you can't mix and match across mazahib (in rukhas) is because many people including scholars are not aware of the length and breadth of the usul and furu3 of the 4 mazhabs, and people can fall into fisq where they're not following any mazhab at all.
Just a crude example - Hanafi fiqh permits marriage without consent of wali for a girl under some conditions. Maliki fiqh permits lack of witnesses under some conditions. Let's say a couple decide to follow both simultaneously and forego permission of wali, as well as witnesses. What you get is outright fisq - a non-marriage and a de facto partnership!
As abu Hasan replied to Alf, at certain times people of one mazhab can take rukhsah from another mazhab following all the stipulated conditions of that mazhab for that matter, consulting a good mufti. Sometimes the permission is widespread. Example, most people in the world today follow the Hanafi ruling for Zakat Al-Fitr payable in cash, as opposed to other mazhabs strictly stipulating giving grains or dates.
Sometimes, people follow one mazhab in one entire bab of fiqh, and another mazhab in another. Mostly otherwise Hanafi Arab and Turkish folks (barring some scrupulous scholars) follow the Shafi3i permissions on sea food (prawns, scallops, mussels, oysters, crabs, octopuses etc.), as eating is a different issue on its own merit. It's not the same as dangerously mixing conditions and rulings across mazahib on an individual matter such as wudu for instance.
The flip side of rukhsah is 3azeemah where you follow the most scrupulous opinion across mazhabs. This is commendable. Example, in Hanafi mazhab gargling the mouth and rinsing the nose is necessary for ghusl, and the flowing of water on every spec of the body is necessary, rubbing by hand not a necessity. In Maliki mazhab, gargling and inserting water in the nose is not necessary; however, rubbing water on the body by hands is a necessity. In Shafi3i mazhab pronouncing the niyah by mouth is a necessity. Now if someone pronounces the niyah by mouth for ghusl, gargles the mouth, rinses the nose, and also rubs water on the body by hand, he is staying in the scrupulous confines of all of Hanafi, Maliki and Shafi3i mazhabs. This is commendable.
So you can't say that the issue of different mazhabs within Ahlus Sunnah is the same things as Sunni/non-Sunni issue in beliefs.
afaik, Ghawthe A3zam radi Allahu 3anhu was well versed with the usul and furu3 of both Hanbali and Shafi3i mazhabs and used to issue rulings from either of the mazhab (depending on the questioner's mazhab). I've heard Mufti Akmal say (not yet read myself) that he was formerly a Hanafi and changed his mazhab to Hanbali as the number of people in the Hanbali mazhab was dwindling in his times.