The Mandalorian Jango Fett was cloned because his genetic material was able to be manipulated by the Kaminoans to create an utterly loyal, fearless, and aggressive soldier. They did not want force-sensitive soldiers because it would take even longer to train them and they might be less tractable. Force sensitives would need to be strong-willed, the last thing you want in your slavishly obedient army.
He was chosen specifically because he was Mandalorian and would be bringing both this genes and his warrior culture ethos to the clones during their training. In addition, with time constraints, it is far easier to work with a genome you are familiar with and modifying it for desirable traits.
Before the gestation process began, the Kaminoans tampered with Fett's DNA to ensure that the clones were primarily dominated by behavioral genes that emphasized certain qualities such as loyalty, aggression, independence and discipline in order to guarantee that the army would be more docile and less independent than their template. Kal Skirata, a former Mandalorian warrior who had been brought to Kamino to assist in the training of a special unit, concurred with the Kaminoans' rationale behind "modified" troopers; an "unaltered" Jango Fett was not the ideal infantry soldier. - Republic Commando, Triple Zero
It is far easier to do significant manipulations on a single genome than to work with thousands of genomes for selective engineering of a better soldier.
Even if the engineers thought they could create something significantly better, it would take both time and significant research to accomplish with no guarantees of quality, ability to control, tendency toward cooperation or other unexpected side effects from merging such varied genetic characteristics.
The genetic potential for a superior soldier was clearly within Jango Fett and with the alterations which included a slavish devotion to the Sith, enabling the execution of Order 66, were included in their genetic make-up. I believe this is the real reason they choose to use the Mandalorian DNA.
As soldiers, the Clone Army (at least as depicted in the televised Clone Wars series) seemed quite effective, especially when augmented with Republic Weaponry, tactics and the superior leadership of their Jedi leader/handlers. The reduction in their capability as soldiers lacking in intuition, versatility and somewhat in reduced intellectual capacity is offset (somewhat) by their ability to coordinate and understand each other's thinking.
And while there is some risk at using a single genome as a template, at no point in the Clone Wars do we see any technology which attacks at a cellular or genetic level making such worries apparently less of an issue.
Another reason biogenic weapons were not used is even as the Republic is working with the Jedi, the Sith knew eventually the Clone Troopers would be working for them. They would not allow such technology to be used, if it could be helped. Facilities which could create such things were likely to be the first places destroyed or blockaded.
At a logistical level, there is the added benefit of having only one specification for all equipment, all tools, all armors and weapons. Facilities producing these materials could create millions of units, quickly and relatively cheaply in comparison to an army with differing anatomical needs.