At 5, you should be able to feel pretty sure about what you are dealing with; he's about the equivalent of a human college student as far as maturity (still capable of having "young and stupid" moments, but if he had an assertive and aggressive personality, you'd certainly know it by now). Sounds like he's a pretty mellow little guy.
As far as the jennet goes, it may be socialization, or it may just be who she is. The donkey/mule hatred for dogs is legendary, yet whether it's all dogs, just unfamiliar dogs, or they really don't seem to give a hoot one way or the other, is to some degree an individual thing. I've talked to people who have had a donkey/mule for years (sometimes several) and never had a problem (some didn't even know that there might be a problem), and I've seen mules that you can't even have a dog on a lead near, because they will go after it, even with the owner right there.
	
		
			
		
		
	
				
			As far as the jennet goes, it may be socialization, or it may just be who she is. The donkey/mule hatred for dogs is legendary, yet whether it's all dogs, just unfamiliar dogs, or they really don't seem to give a hoot one way or the other, is to some degree an individual thing. I've talked to people who have had a donkey/mule for years (sometimes several) and never had a problem (some didn't even know that there might be a problem), and I've seen mules that you can't even have a dog on a lead near, because they will go after it, even with the owner right there.

 
	 
 
		 
			
		
		
		
	
	
			
		 
			
		
		
		
	
	
			
		 
 
		 
			
		
		
		
	
	
			
		 Despite the small size, he is still a donkey in his brain and heart, I would spend as much time working with him as you can, getting to know his personality and his quirks.  Learn equine body language and how they communicate and respond, and it makes it much, much easier to read what is going on with them.  Some warnings are very subtle, some are not, like laid back ears and snorting.  Clicker or marker training works very well with donkeys, they are smart.  My guy is about 750 lbs, does great with the farrier, and in general is pretty easy going.  But he is a donkey, and you just can't forget that.
  Despite the small size, he is still a donkey in his brain and heart, I would spend as much time working with him as you can, getting to know his personality and his quirks.  Learn equine body language and how they communicate and respond, and it makes it much, much easier to read what is going on with them.  Some warnings are very subtle, some are not, like laid back ears and snorting.  Clicker or marker training works very well with donkeys, they are smart.  My guy is about 750 lbs, does great with the farrier, and in general is pretty easy going.  But he is a donkey, and you just can't forget that. 
 
		 
			 
			 
			 
			 
 
		
 
 
		 Too risky IMO.
 Too risky IMO. 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		
 
 
		 
 
		
 
 
		 
 
		 
 
		 
 
		 
 
		 
 
		