IMO, you're making a huge mistake.  You are thinking about how your puppy is going to you along with cats, which is obviously important to your decision BUT the problem is not the age your puppy is now, the fights between your Dobe and your puppy will begin at your puppy gets older.  As your puppy matures he will naturally challenge the Dobe for top position.  There will be fights.  If you are not there to supervise, they could be extremely serious.  They will be serious fights even if you are there but at least then you will be in a position to (hopefully) stop the fight.
My advice is that absolutely under no circumstances should you get a second male, regardless of breed.

Laura Holum
U-CD Dare's Terra Firma, CDX, VC, WAC, HIC, ThD, CGC, TDI
DPCA 2007 Rescue Dog of the Year
UDC 2007 Service Dog of the Year
AKC 2007 ACE Award Honorable Mention
Seneca Exceptional at Lauregen, CD, BH
Ch. Fxhnt Yarrow Continental GT  
Ch. McGypsi's Foolish Indulgence- Gr 1 + more
Tri-Star's Tupelo Honey