Laundrynut
Having used front load machines now for well over 20yrs, I can say that, with one major exception, you can and should mix the size of what you put in and the weight of fabrics (assuming cotton/cotton, synthetic/synthetic).
In almost every load of colours that I do there is at least a pair of jeans, track pants or tops along with much lighter items such as t-shirts, shorts etc and we don't have spin issues.
Two things to consider....
1) avoid washing sheets with bath towels. They can get wrapped together and create a heavy, wet ball....hard to spin and can cause a bad out of balance.
and...
2) Fill the machine. If you are putting less than 1/3 of a drum of clothes in (i.e. 1/3 up the door glass) there may not be enough in it to create a properly distributed load especially if you have only 1 heavy item. Try filling to at LEAST 2/3 up the door (and preferably 3/4 or higher to make the most of the capacity) and you should notice a difference.
Finally, when I bought our first front load machine, an ASEA (Asko today), I was told by the specialist retailer to 'fill it until the drum moves back and forth and I mean right to the top of the drum - that is FULL. Now remove 1 medium sized item, say a polo shirt. You're ready to go.' The drum should only start moving back and forth (as opposed to rotating) if you are putting too much in...or forcing it in.
Following those instructions, neither my mother or I have ever had a problem from ASEA, Hoover, Blomberg, Westinghouse/Electrolux/Zanussi, Fisher and Paykel/Beko or Electrolux.