Why shouldn't different types of bulbs be used in the same fixture?
For a multi-bulb fixture the LED units do not want to be in the area where high heat is generated as would be the case with incandescent or especially with halogen bulbs.

CFLs have their own special considerations if they are used in situations with special types of lamp dimmers. Such special dimmers may not be compatible with LED lighting modules. 

Another consideration comes in from the light quality and perception of what the lighted area looks like. LED lamps have a different light character than do other types of bulbs and mixing them together may be perceived as unpleasant in the extreme cases.