It sounds as if the person who composed the stylebook was not a typophile.
You may have some wiggle room.
If not, You can still use the requested font and comply with the agency stylesheet directive.
What you are trying to accomplish is to make the headings as distinctive as possible within the given restrictions.
Here are a few more things to consider to stay within that typeface "family"…
- All CAPS (bold)
- CAPS and small CAPS (bold)
- Italics (bold)
I would also compose the first paragraph after centred headings to be flush-left to emphasize the break, indenting following paragraphs without a linespace separation (or what is stipulated by the stylebook.)
If you do have some freedom, try all the (probably) sans serif typefaces that come with your software and platform to see which is most appealing to you. Several different typefaces come with MS Word 2010 and Windows 7. As such, you could consider any of those "commonly available"
There is no set "rule" which suggests what mixes with what.