Language / Polish diminuitive names [11]

I would like a little help with this...I work with a lot of Polish men and some of us have been working together for some time. Several of the men were introduced to me with their diminuitive names (Jarek, Jurek, Wojtek). Others not... And there are a number of diminuitives that get thrown around aside from the ones used as introductions. I found this information in the archives:

But beware, different diminutives have different emotional connotations, from more neutrally friendly, to strong affection, to more rough and ready pals. Use a form with -cha with a girl you're interested in and she'll assume you aren't. Use a form ending with -uś with a guy (especially in the vocative) and he may think you are interested in him.

English pet names are nowhere near as interesting or expressive as Polish (or other Slavic) diminutives and English simply cannot express the emotional connotations a well chosen Polish diminutive. They are a linguistic marvel and always repay any interest you show in them.

I have a couple of questions/concerns:

1. One of the diminuitives I've used is a -uś (not knowing the above) with someone that should be shown respect because of his position.

2. I have, on occasion, been referred to as Julka. Not sure what, if any, connotation that may have. Particularly since I have a position that they all seem to have a great degree of respect for... But they all seem very pleased that my given name is Julia and have taken the time to tell me how Polish my name is and how they like it.

3. There are a couple of these men that I now consider friends and I would like to know if it is appropriate to express this feeling using a diminuitive (or another form of the diminuitive) or if it would be best to leave it alone.

Can anyone help me here?